Abstract
Ocean acidification, the decrease in seawater pH due to the absorption of atmospheric CO2, profoundly threatens the survival of a large number of marine species. Cold-water corals are considered to be among the most vulnerable organisms to ocean acidification because they are already exposed to relatively low pH and corresponding low calcium carbonate saturation states (Ω). Lophelia pertusa is a globally distributed cold-water scleractinian coral that provides critical three-dimensional habitat for many ecologically and economically significant species. In this study, four different genotypes of L. pertusa were exposed to three pH treatments (pH=7.60, 7.75, and 7.90) over a short (two-week) experimental period, and six genotypes were exposed to two pH treatments (pH=7.60, and 7.90) over a long (six-month) experimental period. Their physiological response was measured as net calcification rate and the activity of carbonic anhydrase, a key enzyme in the calcification pathway. In the short-term experiment, net calcification rates did not significantly change with pH, although they were highly variable in the low pH treatment, including some genotypes that maintained positive net calcification in undersaturated conditions. In the six-month experiment, average net calcification was significantly reduced at low pH, with corals exhibiting net dissolution of skeleton. However, one of the same genotypes that maintained positive net calcification (+0.04% day-1) under the low pH treatment in the short-term experiment also maintained positive net calcification longer than the other genotypes in the long-term experiment, although none of the corals maintained positive calcification for the entire 6 months. Average carbonic anhydrase activity was not affected by pH, although some genotypes exhibited small, insignificant, increases in activity after the sixth month. Our results suggest that while net calcification in L. pertusa is adversely affected by ocean acidification in the long term, it is possible that some genotypes may prove to be more resilient than others, particularly to short perturbations of the carbonate system. These results provide evidence that populations of L. pertusa in the Gulf of Mexico may contain the genetic variability necessary to support an adaptive response to future ocean acidification.
Highlights
Climate change is dramatically altering the Earth’s terrestrial and aquatic environments, with ocean acidification expected to elicit some of the more severe alterations to marine environments (Keeling et al, 1976; Parmesan and Yohe, 2003; Feely et al, 2004; Hoegh-Guldberg and Bruno, 2010)
We found an overall trend of decreased calcification rate under exposure to undersaturated conditions, but this response took between 2 weeks and 3 months to manifest in the genotypes examined here
It is plausible that the role of the isoform of αCA found in corals (Bertucci et al, 2013), is to regulate the build-up of H+ ions that result from the hydration of CO2 to HCO3− in order to avoid internal acidosis. This has been suggested for the scleractinians Stylophora pistillata (Moya et al, 2008), and the cold water coral Desmophyllum dianthus (Carreiro-Silva et al, 2014), where an over-expression of the genes that encode for CA was found when high levels of H+ were present in the internal compartments of the cells. These data suggest that further ocean acidification will negatively impact L. pertusa reefs in the Gulf of Mexico, transient exposures may not have a significant effect, and some genotypes might be more resilient than others
Summary
Climate change is dramatically altering the Earth’s terrestrial and aquatic environments, with ocean acidification expected to elicit some of the more severe alterations to marine environments (Keeling et al, 1976; Parmesan and Yohe, 2003; Feely et al, 2004; Hoegh-Guldberg and Bruno, 2010). Ocean acidification lowers the saturation state ( ) of calcium carbonate, such that some marine calcifiers are unable to maintain calcification rates that exceed dissolution, or do so at much greater energetic cost. This can compromise the biological and ecological functions of these species, including reduced extent and density of reef framework (Gattuso et al, 1998; Kleypas et al, 1999; Langdon and Atkinson, 2005), failed larval development (Kurihara, 2008), impairment of predator-prey interactions (Cripps et al, 2011; Gaylord et al, 2014), and changes in acid-base regulation (Pörtner et al, 2004)
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